When driving along in cruise control my car has recently started feeling as though it misfires or stutters. It then surges slightly. My gas mileage isn't down at all. It idles fine. City driving doesnt show exhibit the stuttering problem.

69K on a S70 T5, 1999

Could be the dreaded ETM (Electronic Throttle Module) which is $1000 or so to replace (only '99-'01 on the S70). Tiny grooves are worn into the plastic mylar inside the unit. These grooves will be worn where the speed of the car stays constant the longest. Little metal fingers that scrape over the mylar send the signal to the control unit to increase or decrease the throttle and if gooves are formed in the mylar, the signal sent is distorted...Thus the ETM failure could be revealed when using the cruise control if you use it a lot and if it's often set at the same speed....well, at least maybe. Is your cruise control always at about the same speed (If you're like me, 79 in a 70mph highway zone...figuring as long as I don't go 10 miles over the limit I won't be pulled over)? Anyway, a good way to find out is to try to keep the speed at the exact same spot without the cruise control on, just by your foot and see if there is still hesitation. If that's not it, then it could possibly be a small leak in your turbo's vacum line, or could be your A/C switching off and on, like has been mentioned earlier.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon

Do you have the A/C on? sounds like it is cycling on and off which will be noticable with the cruise control on.